Monitoring the Dynamic Response of the Overlying Rock–Soil Composite Structure to Underground Mining Using BOTDR and FBG Sensing Technologies

Deformation and failure of overlying strata caused by underground mining can result in the leakage of overburden aquifers, seriously threatening underground safety and the surface environment. However, conventional methods such as simulation and theoretical calculation cannot comprehensively reflect the dynamic movement of the overburden during mining. This study, therefore, applied two distributed optical fiber sensing (DOFS) technologies, Brillouin optical time-domain reflectometry (BOTDR) and fiber Bragg grating (FBG) sensing technologies, to quantitatively monitor overburden movement in real time. Results indicate that tensile failure dominated in the overburden during mining, and strain accumulation generated mostly at the lithologic interfaces and loess interior. The displacements of bedrock and loess layers reached their maximum values of 76.65 mm and 59.61 mm, respectively, when the excavation face had passed the sensing borehole by 83.16 m. The water-flowing fractured zone (WFFZ) penetrated the bedrock layer and 4.35 m into the loess layer, and it reached the maximum height 225.43 m when the excavation face had passed the sensing borehole by 94.36 m. The fracture angle of roof rock strata was 67.29°. Compared with the microresistivity scanning technology and numerical simulation, the DOFS technologies could more accurately determine the maximum height of the WFFZ. The results are of engineering significance for underground safe mining and surface environmental protection.